It is understood the intruder pushed a security guard to the ground before making off on foot through one of the only windows that opens fully in the building.

Sources believe the burglar may have been trying to remove the Union flag that usually sits on top of Belfast City Hall in a bid to embarrass the council, which was hosting an event to recognise soldiers recently returned from Afghanistan.

A PSNI spokesman said “police attended a report of an unlawful entry of premises at the Donegall Square North area”, and that CCTV footage was being examined as part of the investigation.

It is not clear whether anything was stolen but the incident has sparked outrage among councillors who have called for a “full review” of security at the building.

A similar incident happened in June when a man was found inside the hall by security staff during the early hours of the morning.

He was released when he told them he had been locked in at a function.

However, it was later discovered that the Union flag had been torn down and damaged.

“It is unbelievable,” said Ulster Unionist Jim Rodgers. “I am deeply concerned that this could have happened for the second time in a matter of months. Security must be tightened up.”

It is understood security was not upgraded as part of a £11m refurbishment which was completed in 2009.

Veteran SDLP councillor Pat McCarthy expressed surprise that members had not been informed about the break-in.

“This is the first I have heard about it,” he said. “The councillors have not been told about this by the officers. It is both embarrassing for the city council, and what does it say about how the hall is being managed?”

In a statement a council spokeswoman said: “We can't comment further at this stage as it is now the subject of a police investigation.